at 150 for a 6 volt you could find a used 12 volt generator buy a low buck battery change all your light bulbs and still be ahead of the game and only put in 1 more days worth of work! Then be hapier in the long run!
Batteries Plus. The can get Group 1 and Group 2 6 volts (usually have Gr. 1 in stock). I run them in my Model A and '51 Ford and usually get 2-3 years out of a battery.
Before 12 volt batteries came out, the heavy duty battery was an 8 volt that gave more power and didn't affect the 6 volt system. I don't even know if they make them anymore. I would think a farm supply as has already been mentioned.